Each layer offers its services in the form of primitives.  Four
   classes of primitives are defined as shown in Figure 1.  Request is
   issued by the layer that wants to get the services or the information
   from another layer, and Confirm is the acknowledgment of the request.
   Indication is the notification of the information to the layer that
   requested the service, and Response is the acknowledgment of the
   indication.

   -------------------------   -----------------------------
             Request                       Response
               ||      /             /      ||
   Layer N     ||      ||             ||      ||
   ------------||------||---   -------||------||------------
               ||      ||             ||      ||
               /      ||             ||      /
   Layer N-m        Confirm       Indication
   -------------------------   -----------------------------

                Figure 1: Interaction Model between Layers